Mechanical Properties of Reinforcing Steel and Fatigue Behavior in Corrosive Environment

Corrosion of reinforcing steel has a great impact on the mass reduction and high- and low-cycle fatigue. An experimental study showed that the mass loss, the fatigue limit and the life expectancy were reduced by approximately 1.50-3.00%, 20-40%, and 55-75%, respectively, according to the level of corrosion. Low-cycle strain controlled fatigue testing under ±1% and ±2.5% constant amplitude strain, indicated that the corroded steel bars exhibit gradual reduction in available energy, number of cycles to failure and the load-bearing ability. Formation of pits and notches took place on the corroded steel surface and stress concentration points were developed which are highly localized at imperfections and especially at the rib bases. The fatigue limit was reduced considerably since the existence of ribs and the formation of pits and notches combined with the mass loss led to reduction of the exterior hard layer of martensite and drastic drop in the energy density of the corroded specimens. Antiseismic design that does not take into account the maximum and cumulative plastic deformation demands and the strain history that a structure will suffer under severe ground motion could lead to unpredictable performance.     

Effect of corrosion on mass loss, and high and low cycle fatigue of reinforcing steel

The effect of various levels of corrosion on the mass loss, and the high and low cycle fatigue of BSt500_s steel reinforcement were experimentally investigated. The mass loss, the fatigue limit, and the life expectancy were reduced by 1.5 to 2.9%, 20 to 40%, and 56 to 76%, respectively, according to the corrosion level. Low cycle strain-controlled fatigue testing under a ±1% constant amplitude strain showed that the corroded steel bars exhibit a gradual reduction in the load-bearing ability, the available energy, and the number of cycles to failure. The considerable reduction in the fatigue limit took place because the mass loss led to a reduction of the exterior hard layer of martensite and a drastic drop in the energy density of the corroded specimens, thus developing stress concentration points that are highly localized at imperfections, and especially in the pits and notches of the rib bases of the corroded steel. Because corrosion and cycle fatigue are time-dependent, it seems that steel reliability is also time-dependent.     

Impact of Corrosion on Mass Loss,Fatigue and Hardness of BSt500<Subscript><Emphasis Type="Italic">s</Emphasis></Subscript> Steel

The impact of corrosion on the properties of steel reinforcement in concrete structures was examined. An experimental investigation was carried out in order to gain better insight of the effect of corrosion on the mass loss, fatigue and hardness, of BSt500 _s 12 and 8 mm diameter steel bars that were artificially corroded in a Sodium Chloride environment for different corrosion levels. The fatigue limit of the 12 mm steel was reduced by 20–40% and the mass loss was 1.5–2.9% for 15 and 30 days corrosion level, while the mass loss of the 8 mm steel was 1.2–32% for 10–90 days corrosion. The hardness of the 8 mm steel was reduced by 25–35% and 2–10% in the outer and inner layers of the specimens for 30 and 60 days corrosion respectively. Corrosion created considerable reduction in the fatigue strength and life of the steel bars due to drastic drop in the energy density, formation of pitting and notches along with destruction of the hardest outer layer of martensite.     

Corrosion of reinforcing steel and low cycle fatigue behaviour

Chloride induced corrosion of reinforcing steel can be highly detrimental and of great influence on the low cycle fatigue (LCF) characteristics. An experimental study conducted on BSt500_s reinforcing steel, showed that the LCF and life expectancy were reduced considerably according to the level of corrosion. Low cycle strain controlled fatigue testing under ± 1 and ± 2.5% constant amplitude strain indicated that the corroded steel bars exhibit gradual reduction in available energy, number of cycles to failure and the load bearing ability. Formation of irregularities such as pits, notches and cavities occurred on the corroded steel surface and stress concentration points were developed which are highly localized at imperfections and especially at the rib bases. The experimental investigation of the corroded specimens subjected to LCF showed that the life expectancy, the remaining energy density and the strength properties were reduced considerably as a result of these irregularities combined with the mass loss and reduction of the exterior hard layer of martensite. Structural design capable of resisting seismic activity that does not account for the reduction of the load bearing ability and life expectancy as well as the cumulative plastic deformation of the steel reinforcement due to corrosion and loading history that a structure will be subjected in harsh climatic environments and ground oscillating motion could lead to serious and unpredictable performance.     

Effect of stress corrosion cracking on stress-strain response of steel wires used in prestressed concrete beams

This paper presents an experimental study of the stress corrosion cracking (SCC) process on 8-mm-diameter wires which are used industrially in precast concrete prestressed by pre-tension. The service life of steel wires under accelerated SCC and the reduction of their mechanical performance are studied. A dynamic analysis to detect the damage to corroded wire due to SCC before brittle failure and the influence of internal defects on the service life of stress corroded wire are also presented. The study shows that stress corrosion cracking is characterized by an evolution to SCC from pitting corrosion attacks that result in the development of both micro-cracking and micro-voids in the steel bulk. The stress level does not influence the composition of corrosion products. It is a major factor of SCC development, leading to a considerable reduction in the ultimate strain and thus to brittle failure of the corroded wires. Local defects on the steel surface increase the SCC effect due to stress corrosion concentration. A reduction in the elastic modulus and the elastic limit, which may reach 25% and 15%, respectively, can be expected due to steel micro-cracking. No damage detection through mechanical analysis seems possible before the brittle failure occurs as the corrosion is very localized and so does not globally reduce the tension in the wires.     

An investigation of the reduction in tensile strength and fatigue life of pre-corroded 7075-T6 aluminum alloy

In aging aircraft, the synergetic interaction between corrosion and fatigue has been shown to reduce the life expectancy of aluminum alloys. The objective of this study was to quantify the effects of corrosion, in terms of mass loss per unit area, on the static strength and fatigue life of 7075-T6 aluminum alloy. This was an experimental study in which test specimens were corroded in a laboratory environment. The corrosion process was accelerated by use of a corrosion cell. Test specimens were cut from flat sheets of aluminum and covered with masking material to restrict corrosion to a confined area. After testing, the fatigue life, ultimate tensile strength (UTS), and hardness of the specimens were observed to drop significantly with small amounts of corrosion. After the initial decrease, the UTS was observed to decrease linearly with increasing corrosion levels. The fatigue life of the specimens decreased in an inverse exponential fashion as mass loss per unit area increased. The hardness values of the corroded surfaces were also observed to drop. The topology of the pits and the related subsurface damage produced areas of high stress concentration resulting in the immediate reduction of UTS and fatigue life of the specimens. Subsurface corrosion damage was responsible for the reduction in hardness.     

The corrosion of steel and aluminium in calcium chloride/ammonia,magnesium chloride/methylamine and magnesium chloride/methylamine/decane

The corrosion of St 37, StE 36, Al 99,5 and Al-Mg 3 in the chemical pairs of substances calcium chloride/ammonia, magnesium chloride/methylamine and magnesium chloride/methylamine/decane was investigated. The corrosion tests were performed in autoclaves at room temperature to 180 °C. The nominal duration of the experiments was 1000 hours. Rod shaped fatigue specimens with polished surfaces served as test specimens; these were fatigue tested after the corrosion treatments. All materials tested were compatible with calcium chloride/ammonia under the experimental conditions employed. Steel and aluminium showed similar behaviour against magnesium chloride/methylamine and magnesiumchloride/methylamine/decane, respectively. At room temperature and 70°C to 80°C the corrosion of steel and aluminium was low (wall losses in the order of 1 m?m/a). Corrosion increased with increasing temperature. The activation energy for the reaction, which determined the rate of weight loss in magnesium chloride/methylamine/decane, was AE_st = 0, 60 ± 0,18 eV for steel, and AE_Al = 1, 07 ± 0,07 eV for aluminium. A corrosion treatment on aluminum fatigue specimens after 1000-1500 hours at 110 °C caused a notable decrease in fatigue strength; no such behaviour was noted for steel, even when corroded at 170 °C.     

P110钢在CO2／H2S环境中的适用性研究

采用高温高压实验设备辅以失重法,研究了CO2／H2S腐蚀环境中P110钢的腐蚀性能,用SEM、EDS和XRD等分析了腐蚀产物．分别用电化学充氢及NACE TM0177A法对P110钢进行耐氢损伤试验．结果表明,虽然P110钢在试验环境中的均匀腐蚀速率很小,未发生点蚀,但随着充氢量的增加,强度、伸长率及断面收缩率均降低．...     

Electrochemistry of deformed smooth surfaces and short corrosion fatigue crack growth behaviour

Abstract

The polarisation characteristics for a deformed smooth surface of a 0·2% carbon steel in an artificial sea water have been determined under static and cyclic loading conditions. The influence of strain level and loading frequency on anodic and cathodic Tafel constants, corrosion current density, and corrosion potential is described. Based on these data, the conditions for corrosion fatigue testing which corresponded to a maximum synergism between surface deformation and anodic dissolution were determined. Corrosion fatigue tests, which were conducted under both constant potential and constant current density conditions have shown that a process of metal dissolution plays a determining role in the short corrosion fatigue crack growth behaviour. An experimentally based criterion is proposed involving the development of a short corrosion fatigue crack, of characteristic size, which is associated with the spacing between the major microstructural barriers. This criterion is a function of both shear stress and the parameters controlling the anodic dissolution process on a cyclically deformed smooth suiface. An expression predicting the formation of short corrosion fatigue cracks is presented which takes into account the synergistic action of shear stress and the corrosion process.     

Versprödungserscheinungen an ferritischen Chromstählen und deren Einfluß auf das Schwingungsrißkorrosionsverhalten

Embrittlement conditions of ferritic chromium steels and influence on the corrosion fatigue behaviour The corrosion fatigue behavior of two ferritic stainless steels, with 18% and 28% chromium respectively, embrittled by α'- or sigma-phase has been studied in this investigation. No effect of the α'-phase on the corrosion fatigue behaviour in a 0.5 N or 4 N air saturated soldium chloride solution at 80 °C has been found. In the 4 N solution corrosion fatigue cracking of the 18% chromium steel in the solutionized as well as in the α'-embrittled condition is initiated by pitting at nonmetallic inclusions. Sigma-phase not only decreases the mechanical properties of the 28% chromium steel by heavy embrittlement but also leads to crack initiation by pitting in the 4 N solution at the interface sigma/ferrite. The pronounced decrease of the corrosion fatigue properties of the 28% chromium steel could well be correlated with chromium depletion of the ferrite matrix by precipitation of sigma phase.     

The Impact of Corrosion on the Mechanical Behavior of Welded Splices of Reinforcing Steel S400 and B500<Subscript>c</Subscript>

The reinforcing steel, used in concrete structures, when corroded causes reduction of the strength properties and especially drastic reduction of ductility. Steel corrosion constitutes an important factor of progressive devaluation of its mechanical properties and serious reduction of the integrity of structures. The problem becomes more evident specifically for structures near coastal areas where salt corrosion is predominant. Reinforced concrete columns and beams are quite often extended by welding new steel reinforcement to the already corroded existing steel. In the present article the impact of corrosion on the mechanical properties of welded splices of reinforcing Steel S400 and B500_c is examined. An experimental investigation was conducted and tensile and compressive results are presented for welded precorroded S400 and noncorroded B500_c steel splices. The mechanical behavior of welded splices in tension are different in compression and depend strongly on the level of corrosion of the S400 bars.     

FATIGUE PROPERTIES OF AUSTEMPERED DUCTILE IRON （ADI） IN WATER ENVIRONMENT

The acicular ferrite in austempered ductile iron (ADI) matrix around graphite was corroded preferentially in wet condition, promoting crack origination and propagation and resulting in the disappearance of ADI fatigue limit. ADI fatigue strength was gradually reduced with increasing the time of test and was reduced by 50% in wet condition at 10^7 cycles compared with the fatigue limit in dry condition.The fatigue strength variation of ferritic ductile iron in wet condition was similar to that of ADI.The ferritic ductile iron, however, has better corrosion resistance so that the fatigue strength was lowered only by 10^7 in wet condition at 10^7 cycles compared with the fatigue limit in dry condition. On the other hand, the fatigue limits of ADI and ferritic ductile iron were dropped by 32% and 25% in tap water dipping 480h/dry condition respectively compared with those in dry condition.The reduction of fatigue limit was attributed to corrosion pits formation correlated with stress concentration, resulting in origination and propagation of fatigue crack.     

Korrosionsbedingte Veränderungen von Festigkeitseigenschaften metallischer Werkstoffe

Changes in the strength properties of metallic materials due to corrosion Corrosion produces changes in the strength properties of metals and such changes can be assessed by mechanicophysical testing of materials. The residual strength after corosion is proportional to the weight loss, e.g. for uniformly corroded copper and when the specimens are tested in the tensile ruptured. The elongation at break, however, shows an unexpectedly strong decrease already at low corrosion levels. In the case of intercrystalline corrosion the strength properties depend not only from the reduction of the load-bearing cross section, but, additionally, are determined by the notch effect of corroded grain boundaries. This fact is shown on potentiostatically precorroded specimens of a CrNi steel 18/9. At transpassive potentials this material is attached by intercrystalline corrosion even after solution annealing. Because oft he low intensity of the intercrystalline attack in this range of potentials there are, however, no significant differences in the strength behaviour of the steel after different sensibilization treatments. The fatigue resistance of the CrNi steel is becoming worse as intercrystalline corrosion is advancing into the depth of the metal structure. Chromium carbide precipitation have a favourable effect because they block slip planes and thus improve fatigue resistance.     

Effect of electrochemically reactive rust layers on the corrosion of steel in a Ca(OH)2 solution

Electrochemical and gravimetric measurements are used to study the effect of rust layers on the corrosion of rusted steel in a saturated Ca(OH)₂ solution imitating the liquid phase in concrete pores. The results indicate that the reduction of rust is the main cathodic reaction in the first phase of the corrosion process of rusted steel. Subsequently the oxygen reduction reaction becomes the cathodic predominant reaction, in which the rust seems to act as a porous electrode. In this research rust clearly favours the corrosion of rusted steel, and an approximately direct correlation is seen between the rust mass and the mass loss due to steel corrosion. In the electrochemical measurements, attention is paid to the overestimation of the corrosion rate deduced from the polarisation resistance in the case of specimens with the highest rust contents, a problem that is attributed to the inappropriateness of the conventional measuring procedure in the presence of high capacitance values and redox processes.     

The Influence of Corrosion and Cross-Section Diameter on the Mechanical Properties of B500c Steel

Corrosion is a negative contributor on the structural integrity of concrete structures and leads to degradation of the mechanical properties of steel rebar. Exposure to chloride, seawater, salt and saltwater and deicing chemical environments influences the concrete-steel bond and weakens it. A considerable strength factor of the two-phase steel B500_c (martensitic, ferritic-perlitic) is considered to be the outer martensitic cortex thickness, which varies according to the area of the rebar cross section. In order to evaluate the influence of corrosion and the size of the area on the mechanical properties of B500_c steel, an experimental investigation was conducted on B500_c ribbed steel rebar of 8, 12, 16, and 18 mm diameter, and which were artificially corroded for 10, 20, 30, 45, 60, 90, and 120 days. The laboratory tests suggest that corrosion duration and rebar cross-sectional area size had a significant impact on the strength and ductility degradation of the specimens. The tensile mechanical properties before and after corrosion indicated progressive variation and drastic drop in their values. The extended salt spray exposure enhanced the damage and created pits and notches, resulting in stress concentration points and progressive reduction of ductility and available energy. Anti-seismic design and codes that ignore the influence of the size of the cross-section area and the level of corrosion and mechanical behavior of reinforcing steel could lead to unpredictable performance during severe ground motion.     

Effect of corrosion severity on fatigue evolution in Al-Zn-Mg-Cu

The effect of existing-localized corrosion on fatigue cracking of 7075-T6511 was established using crack surface marker-band analysis and a fracture mechanics model. The substantial reduction of fatigue life due to EXCO solution L-S surface pre-corrosion is nearly independent of exposure time after initial-sharp degradation, scaling with the evolution of pit-cluster size and initial stress intensity range with exposure time. Independent of exposure time, formation of a resolvable fatigue crack (∼10 μm) accounts for a similar-low (∼5%) fraction of total fatigue life at low stress range (σ_max = 150 MPa, R = 0.1). Crack formation occurs at microscopic protrusions into the corroded volume. A corrosion-modified-equivalent initial flaw size (CM-EIFS); predicted with the AFGROW tool using measured initial aspect ratio, initiation cycles, and total fatigue life inputs; accurately represents the corrosion damage effect on fatigue for a range of exposures. The similar deleterious effect of several corroding environments for various-exposed surfaces is described by a lower-bound CM-EIFS with a 300 μm depth and 1200 μm surface length suggesting fatigue is governed by a microscopic pit-based topography. Either an approximate lower-bound, or specific CM-EIFS calibrated by limited measurements of fatigue life for service-environment exposed specimens, can be used to assess the impact of corrosion in a damage tolerant framework. Complexities (e.g., local H embrittlement, 3D pit geometry, topography dependent initiation, and microstructure sensitive small-crack growth) do not compromise the CM-EIFS estimation, but must be better understood for refined modeling.     

Low cost corrosion damage mitigation and improved fatigue performance of low plasticity burnished 7075-T6

Low plasticity burnishing (LPB) has been investigated as a surface enhancement process and corrosion mitigation method for aging aircraft structural applications. Compressive residual stresses reaching the alloy yield strength and extending to a depth of 1.25 mm (0.050 in.) deeper than typical corrosion damage is achievable. Excellent surface finish can be achieved with no detectable metallurgical damage to surface and subsurface material. Salt fog exposures of 100 and 500 h reduced the fatigue strength at 2×10⁶ cycles by 50%. The LPB of the corroded surface, without removal of the corrosion product or pitted material, restored the 2×10⁶ fatigue strength to greater than that of the original machined surface. The fatigue strength of the corroded material in the finite life regime (10⁴ to 10⁶ cycles) after LPB was 140 MPa (20 ksi) higher than the original uncorroded alloy and increased the life by an order of magnitude. Ease of adaptation to computer numerical control (CNC) machine tools allows LPB processing at costs and speeds comparable to machining operations. Low plasticity burnishing offers a promising new technology for mitigation of corrosion damage and improved fatigue life of aircraft structural components with significant cost and time savings over current practices.     

高速列车车轴用EA4T钢腐蚀疲劳性能研究

目的 研究腐蚀环境中EA4T车轴钢疲劳性能,为车轴的腐蚀检测和使用寿命评估提供依据.方法 采用旋转弯曲疲劳试验机,在人造雨水模拟的腐蚀环境和空气环境中,对EA4T车轴钢试样进行疲劳试验,以获得不同环境下试样的疲劳S-N曲线、表面损伤以及裂纹扩展规律.然后对扩展裂纹进行概率统计,通过扫描电镜对疲劳失效的断口进行观察,并分析对比不同环境中裂纹扩展门槛值的变化.结果 空气环境中,试样的疲劳极限为355 MPa,而在腐蚀环境中,试样不存在疲劳极限,107循环周次对应的疲劳强度降低到245 MPa,相比空气环境中降低了31％.Gumbel分布统计与Weibull双参数分布统计相比,更适合描述EA4T车轴钢试样表面腐蚀裂纹长度随加载次数的变化.腐蚀环境中,疲劳裂纹萌生于表面腐蚀坑,并存在多个裂纹源.腐蚀环境显著降低了试样裂纹扩展门槛值,空气环境下,该值为6.29 MPa·m1/2,腐蚀环境下降低到4.1 MPa·m1/2.结论 腐蚀环境降低EA4T钢疲劳寿命的主要原因是,腐蚀环境降低了裂纹扩展门槛值,加快了裂纹萌生以及短裂纹扩展.而当裂纹达到一定长度时,腐蚀环境对裂纹扩展几乎没有影响.     

植入铸造不锈钢髋关节断裂失效事例分析

利用ＳＥＭ对实际使用时断裂的髋关节的断口和表面进行了分析，认为此关节的断裂为以弯曲应力为主的腐蚀疲劳断裂。疲劳裂纹首先在表面的腐蚀点孕育产生，应力和腐蚀的共同作用使得裂纹得以扩展断裂。     

Potentialabhängigkeit der Schwingungsrißkorrosion von höherfestem Dalbenstahl in Salzwässern

Potential dependence of the corrosion fatigue of high strength sheet piling steel in salt water Cathodic protection of high strength steel, grade X 65, against corrosion fatigue (25 Hz) is not effective in seawater. An influence of hydrogen is assumed because hydrogen embrittlement occurs at slow strain-rates (not a constant load) under the condition of cathodic hydrogen evolution. The potential dependency of corrosion fatigue at 5 Hz was investigated with notched and smooth electropolished tensile specimens. The notched specimens showed a slight decrease of the number of cycles to failure with decreasing potential. The polished smooth specimens displayed cathodic protection. The results of the investigation can be explained by recent publications as an participation of hydrogen embrittlement, which happens only at critical strain rates and is dependent on the amount of straining, the frequency and the surface of the specimen.